Subscriber line interface circuits (SLICs) are often present in a central office exchange of a telecommunications network or remote locations thereto for use in providing a communication interface between a digital switching network of a central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location that is remote from the central office exchange.
The analog subscriber line and subscriber equipment (e.g., a telephone) form a subscriber loop. The interface requirements of a SLIC typically require high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are typically low voltage analog signals on the subscriber loop. Accordingly, the SLIC performs various functions with respect to voiceband and control signaling between the subscriber equipment and the central exchange.
SLIC functionality has generally been implemented in multiple integrated circuits (ICs), or combinations of ICs and discrete elements. Typically, significant high voltage circuitry is included in one IC to provide various high voltage functionality of a SLIC. Accompanying low voltage IC's are used to perform control functions for the high voltage portion and also to perform low voltage tasks, voice signal processing, and to provide an interface to system circuitry, e.g., a system on a chip (SOC) such as a digital signal processor (DSP) or other digital processing circuit of a central office or similar location. In turn, the DSP is coupled to provide system input/output (I/O) signals to other locations in the telecommunications network. In other implementations, instead of a DSP interface, the SLIC may couple directly into a switching system.
Typically, a significant number of wires or signal lines are used to connect low voltage portions of a SLIC with the high voltage portion. Furthermore, different SOCs or DSPs used in a system can require different information from a SLIC. That is, different DSPs have different capabilities with respect to signal processing. Some DSPs include capabilities for analog signal processing such as codec functionality and filtering, while other DSPs strictly handle digital signal processing for system requirements such as code compression, call processing, echo cancellation, among others. Accordingly, different SLIC configurations are needed to interface with different DSPs.
These different SLIC configurations typically require completely different designs, often in different process technologies. Such different designs are not readily reused across different process technologies and different SLIC configurations. Another limitation with respect to SLIC design is that because of the criticalities of the different low voltage and high voltage components, it is typically difficult to port a given design across different process technologies. Thus, a SLIC design implemented in one process technology is not easily ported to another technology, owing to differences in device characteristics. This typically requires the need for significant calibration, trimming and other design-intensive matching of devices.
To power the SLIC as well as equipment on the subscriber loop, SLICs typically implement one or more tracking voltage regulators. These regulators operate by measuring the voltage across the loop and adding overhead voltage. In this way, a regulator may track the line voltage, and provide sufficient extra voltage for the electronic circuits of the SLIC to operate properly. Typically, such regulators are implemented using high voltage circuitry, which consumes excessive space, power and increases heat dissipation.
Switching regulators can suffer from slow transient response to changes in line conditions. For example, such transient response may occur when subscriber equipment connected to the SLIC is placed off hook, in ringing, and so forth. For example, going from off hook to on hook may cause an overload of a DC feed loop, causing dial pulse distortion or other undesired effects. Furthermore, when multiple pieces of subscriber equipment are coupled to a line, transient response can be negatively affected. Typical regulators incur time delays in responding to such transients, leading to deleterious effects on performance.